A power converter provides power by converting power from a source into a form suitable for a load of interest. For instance, a power converter can provide DC power from an AC source. An acceptable commercially viable power converter needs to ensure that the Electromagnetic Interference (EMI) generated by its operation does not exceed acceptable levels. While there are many mechanisms responsible for the generation of EMI, a well-known component of EMI is common mode noise. A switching power converter generates common mode noise as a result of the switching operations in the presence of a low impedance path to ground. Typically, common mode noise due to common mode current flow makes up a significant fraction of the electromagnetic interference (EMI) generated by a switching power converter.
In a switching power converter, the switching circuit receives input power from the input terminals and then produces a switching waveform across the main transformer. The switching waveform so produced is coupled through inter-winding capacitance as well as secondary winding to the secondary side. This secondary winding feeds power to the rectifying circuit which in turn produces power to the load.
In addition to the above described power distribution, there is a path for common mode current responsible for the common mode noise. Switching operations generate noise, which is coupled through transformer inter-winding capacitance to the secondary side. In general the load is isolated from earth but it has fairly high capacitance coupled to earth. This capacitance, together with transformer inter-winding capacitance provides a path for common mode current to flow through the power source impedance by completing the circuit. Any current through the detected power source impedance contributes to conducted electromagnetic interference. The complete common mode current path has a noise source coupled with inter-winding capacitance, which, in turn, forms a complete loop with the power source impedance and the parasitic capacitance on the load side.
Reduction of common mode noise in switching power converters presents a difficult problem. In particular, a switch mode power converter with isolation transformers presents numerous challenges. Usually this type of power converter has close coupling between the primary and secondary windings. Such close coupling reduces leakage inductance and improves conversion efficiency. However, the close coupling, i.e., high transformer coupling coefficient, increases inter-winding capacitance between the primary and the secondary windings and this increases undesirable noise coupling from the primary side to the secondary load side.
Typically, a bypass capacitor connecting two “non-switching” nodes on the primary side and the secondary side respectively reduces the common mode current. A usual choice of nodes is one of the input terminals and the secondary common node. This reduces the common mode current coupled through parasitic capacitance between the load and earth. However this method has its limitations. Safety standards prevent use of high-capacitance because this will increase leakage current between the primary and secondary side.
Placing a sheet of shielding metal known as the “Faraday shield” between the secondary and primary windings also reduces the common mode current. This strategy works on the same principle as the bypass capacitor and provides an additional shunt path for the noise current. It effectively provides another capacitance path in parallel with the bypass capacitor. However, the shield makes the transformer very bulky and reduces magnetic coupling between primary and secondary windings. In turn, this reduces the converter efficiency—an undesirable outcome.
Yet another commonly known method exploits passive filtering by making use of bulky filtering components to suppress noise. This method is widely used but is becoming increasingly undesirable due to the additional components required and the resulting large size of the device.
In U.S. Pat. No. 6,137,392, Edward Herbert's invention uses two or more transformers connected in series to reduce the overall parasitic capacitance between primary and secondary windings. This approach also requires additional magnetic components and tedious magnetic component construction. Moreover, theoretically this approach cannot completely eliminate noise coupling through the isolation transformer.